Actuation systems may be used, for example, in order to actuate hydraulic or pneumatic pistons and to move them into a desired position. Normally, these are safety-relevant applications. Consequently, it is necessary to use an actuation system with a safety position, whereby, in case of a failure of the actuation system, the safety circuit moves the actuator into a safe position, that is to say, the safety position.
Such an application could be, for instance, the blade adjustment of a wind turbine. In this application, it has to be ensured that a safety function is provided if the regular adjustment system fails. It has to make it possible to adjust the rotor blades in such a way that no uncontrollable operative states can occur. It would be likewise conceivable to use such actuation systems for gas, steam or water turbines as well as for applications in the oil and gas industries, where actuators are likewise employed which, in case of a failure, have to be safely and reliably moved into a prescribed position. Such actuators could be, for example, hydraulic pistons or safety valves.
International Patent Application Publication No. WO 2009/064264 A1 discloses an electro-hydraulic circuit used to set the angle of attack of at least one rotor blade of a wind turbine. The circuit comprises a motor that drives a pump. The hydraulic circuit is supplied with fluid from a tank or reservoir that can be pumped into a first or second chamber of a hydraulic piston by means of the pump. The direction of rotation of the pump is variable, whereby, in a first direction of rotation, fluid is pumped into one of the chambers, and the fluid displaced out of the other chamber is directed to the suction side of the pump and is likewise conveyed to the chamber that is to be filled. The hydraulic circuit also comprises a pressure tank that is filled with hydraulic fluid and that is connected to the hydraulic circuit via a spring-loaded valve. During the regular operative state, the tank is closed by the valve and is thus decoupled from the hydraulic circuit. In case of a failure, for example, a power failure, the spring-loaded valve is moved into the through-flow position, as a result of which the hydraulic fluid stored under pressure in the tank enters the circuit. The valves, which are positioned in the feed lines of the two chambers of the hydraulic piston, are moved into the through-flow position by means of spring force. The hydraulic fluid stored in the pressure tank then flows through the hydraulic circuit into both chambers of the hydraulic piston. In this process, the fluid flows out of the tank into the working circuit, that is to say, into the circuit that supplies the piston with fluid during regular operation. The fluid can flow via two routes through the working circuit in the direction of the chamber of the piston that is to be filled, namely, through the pump and/or through a line in which there is a throttle as well as a switching valve that is open in the failure state. The hydraulic piston is configured as a so-called differential piston, which is why, in spite of the fact that the pressure in the two chambers is identical, a greater force is exerted on the larger of the two piston surfaces, causing a corresponding movement of the piston into the safe position The drawback here is that, if the pump is blocked or if the line or the circuit elements are defective, this circuit cannot ensure a fail-safe function.
A fail-safe actuation system of this generic type is known, for example, from U.S. Pat. No. 5,301,505. A pressure tank is used so that the cylinder that can be actuated by two chambers can be moved into a safety position. A comparable construction is also disclosed in European Patent Application EP 1498614 A1.
Therefore, it is the objective of the invention to put forward a fluid-based actuation system that, in the failure state, can reliably assume a prescribed safety position.